This invention relates to improved catalysts for the oxidation of carbon monoxide and methods of preparing these catalysts. The catalysts of this invention convert carbon monoxide, produced by the combustion of carbonaceous heat sources, to a benign substance such as carbon dioxide. The catalysts and methods of this invention are particularly suitable for use in a smoking article such as that described in copending U.S. patent application Ser. No. 223,153, filed on Jul. 22, 1988, now U.S. Pat. No. 4,991,606, (PM-1322) and commonly assigned herewith.
The catalysts of this invention comprise mixed transition metal oxide catalysts and catalyst supports. According to the methods of this invention, the catalysts are prepared using a sequential precipitation process which generates catalysts that contain substantially layered metal oxides, and, in some embodiments, a noble metal or mixtures of noble metals layered on the metal oxides.
There have been previous attempts to provide catalysts containing metal oxides and mixed metal oxides alone and in combination with noble metals or their oxides for the oxidation of carbon monoxide to carbon dioxide. There have also have been previous attempts to provide methods for the manufacture of such catalysts. These attempts have not produced catalysts having all of the advantages of the catalysts of the present invention.
For example, Callahan et al. U.S. Pat. No. 3,546,138 refers to oxidation catalysts that consist of a base catalyst containing the mixed oxides of antimony and iron, on a silica carrier, formed by coprecipitation. The formation of these catalysts is facilitated by a metal oxide promoter, which is incorporated into the base catalyst by coprecipitation or impregnation.
Haruta et al. U.S. Pat. No. 4,698,324 refers to a method for the manufacture of a composite catalyst having gold or a mixture of gold and an oxide of chromium, manganese, iron, cobalt, nickel, or copper, which is deposited on a carrier by coprecipitation. The method requires that urea and/or acetamide be used as a precipitant to facilitate the deposit of the gold/metal oxide mixture on a support in a single step reaction.
Haruta et al., Journal of Catalysis, 115, pp. 301-09 (1989), Haruta et al., Proceedings--International Congress on Catalysts, 3, pp. 1206-1313 (1988), and Haruta et al., Chemistry Letters, 2, pp. 405-408 (1987), refer to catalysts containing gold in combination with a single transition metal oxide that are prepared using a coprecipitation method.
Haruta, et al., Advanced Hydrogen Energy, 2, pp. 1135-47 (1981), refers to mixed transition metal oxide catalysts used in the catalytic combustion of hydrogen.
Bond et al., Chemistry and Industry, pp. 878-79 (1967), refers to mixed noble metal oxides for use in hydrogenation reactions. The mixed noble metal oxides contain platinum and either iron, cobalt, nickel or copper, or palladium and either cobalt or nickel.
Bond et al. United Kingdom patent 1,134,111 describes homogeneous catalyst mixtures comprising a platinum group metal oxide and a base metal oxide prepared by the fusion of the mixed salts to give the mixed oxides.
European patent application 0 130 835 describes composite mixed metal oxide catalysts of lanthanum, neodymium or praseodymium, or mixtures thereof, supported by aluminum oxide, prepared by a coprecipitation or an impregnation method.
Japanese patent publication no. Sho 61/1986-227842 refers to carbon monoxide removing catalysts that are all based on the presence of manganese dioxide and palladium. These catalysts achieve less carbon monoxide oxidation than the catalysts of the present invention.
Japanese patent publication no. Sho 50/1975-36387 refers to hopcalite catalysts consisting of manganese or manganese oxide that are prepared using a coprecipitation method.
Japanese patent publication no. Sho 52/1977-139684 refers to mixed metal oxide catalysts prepared through the decomposition of a combination of acetates.
Japanese patent publication no. Sho 54/1979-112793 refers to mixed metal oxide catalysts prepared using coprecipitated Fe.sub.2 O.sub.3 /Al.sub.2 O.sub.3 on which a catalytically active metal is supported.
Japanese patent publication no. Sho 63/1988-252908 refers to ultra-fine gold particles which are supported by a metal oxide, prepared by the reduction of a gold salt in the presence of the metal oxide, a procedure commonly used to prepare metal catalysts supported on metal oxides.
These previous attempts differ from the methods of the present invention in that they use coprecipitation to prepare the catalysts, not sequential precipitation. As a result, the catalysts produced do not have the chemical properties of the catalysts of the present invention and, accordingly, do not have all of the advantages of the present invention.